Abstract

In this thesis is presented a brief review of quantum computing, the DiVincenzo criteria, and the possibility of using a solid state system for building a quantum computing architecture. Donor electron systems in silicon are discussed, before chalcogen, \deep", double donors are suggested as a good candidate for fulfilment of the criteria; the optically driven Stoneham proposal, where the spin-spin interaction between two donor electron spin qbits is mediated by the optically controlled, excited, state of a third donor electron, forms the basis of this [1]. Coherence lifetimes are established as a vital requirement of a quantum bit, but radiative lifetimes must also be long. If the spin-spin interaction between qbits is decreased, or turned off, by the de-excitation of the mediating donor electron then the coherence of the qbit is rendered irrelevant; de-excitation will ruin quantum computations that depend upon an interaction that only happens when the mediating electron is in an excited state.
Effective mass theory is used to estimate excited state donor, 2P, wavefunctions for selenium doped silicon, and recent Mott semiconductor to metal transition doping data [2] is used to scale the spatial extent of the 1S(A1) ground state wavefunction. Using these wavefunctions, the expected radiative lifetimes are then calculated, via Fermi's golden rule, to be between 9 ns and 17 ns for the 2P0 state, and 12 ns to 20 ns for the 2P_1 state. Fourier Transform InfraRed (FTIR) absorbance spectroscopy is used to determine the optical transitions for selenium donors in silicon, this has allowed agreement between literature, measured, and effective mass theory energy values for the particular samples measured. FTIR time resolved spectroscopy has then been used to measure the radiative emission spectrum of selenium doped silicon samples at 10-300K, following a 1220 nm laser pulse. Fitting to the exponentially decaying emission data, selenium radiative lifetimes as long as 80 ns are found; for the 2P0 to 1S(A1) transition in an atomic selenium donor complex at 10K. A factor of between 4 and 8 agreement is found between calculated and measured radiative lifetimes. This offers the possibility of nanosecond scale donor electron coherence times for chalcogen dopants in silicon.